1. Field of the Invention
The present invention relates to a stack-type piezoelectric element, and relates to a structure of an overall stacked piezoelectric actuator which is excellent in moisture durability, reliability and productivity and which particularly can work at low operating voltage.
2. Description of the Prior Art
Piezoelectric ceramics have a property to perform mutual conversion of mechanical energy and electrical energy, so that the application of the ceramics to sensors and actuators for detecting dynamical quantities has been actively developed. However, since the percentage of elongation of piezoelectric ceramics per unit voltage applied is low, very high voltage must be applied to obtain a practically useful displacement. Accordingly, a method is frequently used to lower the voltage to be applied which uses a structure wherein a large number of piezoelectric ceramic sheets of as small a thickness as possible are stacked. Piezoelectric elements having such a structure are called stack-type piezoelectric elements.
One of the most advanced technologies for production of stack-type piezoelectric elements is the green sheet method. In this method, powders of a piezoelectric material are dispersed in a suitable solvent, the resultant slurry is made up into the form of sheet, and a metal paste is coated on the sheet to serve as an electrode by means of screen printing, etc. A large number of the sheets thus obtained are stacked and pressure bonded, then dried and sintered. By virtue of the above method of production, it has become possible to lower the applied voltage necessary to obtain a maximum elongation of 0.1% down to 100 V, which value is commercially accepted as the maximum elongation of piezoelectric elements. Stack-type piezoelectric elements obtained by such prior art green sheet method are already known, for example, from Japanese Patent Publication No. 59-32040 and the article described in Sensor Gijutsu (Technology) 3, No. 12, p. 31 (1983).
Further, as an electrode structure which reduces the internal stress of a stack-type piezoelectric element and improves its reliability, there is described a system wherein the area of the piezoelectric ceramic and that of the electrode are made equal (hereinafter referred to as overall stacked system) in Japanese Patent Publication No. 63-17354. The cross-section of the overall stacked piezoelectric element is shown in FIGS. 8 and 9. FIG. 9 is a sectional view of the element of FIG. 8 taken along line A--A'.
As other examples of prior art related to the present invention, mention may be made of Japanese Patent Application Kokai Nos. (Laid-open) 62-62,571, and No. 61-27,688 etc.
In the former of the prior art methods described above, the sintering of the green sheet of the piezoelectric ceramic sheet 1 and the stacking thereof with the electrode 2 are simultaneously performed at about 1300.degree. C. Therefore, noble metals stable at high temperature, particularly silver-palladium (AgPd) alloy, have been commonly used as the material for the electrode 2. However, piezoelectric elements using said silver-palladium alloy electrode have the problem of a short-circuit between electrodes occurring during operation in humid atmosphere. The cause for this is conceivably that water permeates the molding resin 5 which is to protect the side face of the piezoelectric element, and a layer of water is formed between the stacked body and the resin 5, resulting in a low contact resistance between electrodes, or causing silver to dissolve out as ions (Ag+) thereinto, which are attracted by the electric field and deposited in the vicinity of neighboring electrodes, forming a current-carrying path. This phenomenon is generally called "migration" and is a problem common to all electrical parts using silver or silver-base alloy electrodes.
On the other hand, in overall stacked piezoelectric elements, the latter of the prior art technologies, a method is employed wherein the insulator 4 is coated along the end face of the electrode 2 in order that the electrode 2 and the lead-out terminal 3 are connected alternately, as shown in FIG. 8. In this method, however, the width and the thickness of the insulator 4 must be sufficiently large as compared with the thickness of the electrode 2 so as to ensure necessary dielectric strength, which becomes an obstruction in decreasing the distance between electrodes or the thickness of the piezoelectric ceramic sheet 1, resulting in difficulty in decreasing the operating voltage. Further, since the terminal 3 is formed on the rugged surface of the insulator 4, it causes a problem in its reliability. Another problem is that since the production steps are complicated, the yield of the product is low and the cost of production is high.